1. Field of the Invention
The invention relates to methods and systems for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improvements in vehicle fuel economy.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituents, including hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). The respective rates at which an engine generates these constituents are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for various of these xe2x80x9cfeedgasxe2x80x9d constituents based, for example, on detected values for instantaneous engine speed and engine load.
In order to comply with modern restrictions regarding permissible levels of selected exhaust gas constituents, vehicle exhaust treatment systems often employ one or more three-way catalysts, referred to as an emission control device, disposed in an exhaust passage to store and release selected exhaust gas constituents, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Significantly, a device""s actual capacity to store a selected constituent gas, such as NOx, is often finite and, hence, in order to maintain low tailpipe NOx emissions, the device must be periodically cleansed or xe2x80x9cpurgedxe2x80x9d of stored NOx. The frequency or timing of each purge event must be controlled so that the device does not otherwise reach its actual NOx storage capacity, because engine-generated NOx would thereafter pass through the device and effect an increase in tailpipe NOx emissions. Further, the timing of each purge event is preferably controlled to avoid the purging of only partially-filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture and, particularly, the fuel penalty associated with the release of oxygen previously stored in any other upstream emission control device.
In response, U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153 teach use of NOx-estimating means which seeks to estimate the cumulative amount of NOx which has been generated by the engine and, presumptively, has been stored in the device during a given lean operating condition. The incremental amount of NOx believed to have been generated and stored in the device is obtained from a lookup table based on engine speed, or on engine speed and load (the latter perhaps itself inferred, e.g., from intake manifold pressure). However, the disclosed NOx-estimating means fails to account for any instantaneous reduction in device efficiency, i.e., the device""s ability to store an additional amount of feedgas NOx. The disclosed NOx-estimating means further fails to account for the device""s initial storage of oxygen which likewise reduces the device""s overall NOx-storing capacity.
The prior art has also recognized that the device""s actual or maximum capacity to store selected exhaust gas constituents is often function of many variables, including device temperature, device history, sulfation level, and thermal damage, i.e., the extent of damage to the device""s constituent-storing materials due to excessive heat. See, e.g., U.S. Pat. No. 5,437,153, which further teaches that, as the device approaches its maximum capacity, the incremental rate at which the device stores NOx may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx capacity which is significantly less than the actual NOx capacity of the device, to thereby theoretically provide the device with a perfect instantaneous NOx-storing efficiency, i.e., the device stores all engine-generated NOx, as long as stored NOx remains below the nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the nominal device capacity. Unfortunately, however, the use of such a fixed nominal NOx capacity necessarily requires a larger device, because this prior art approach relies upon a partial, e.g., fifty-percent NOx fill in order to ensure retention of engine-generated NOx.
When the engine is operated using a fuel containing sulfur, SOx accumulates in the device to cause a decrease in both the device""s absolute capacity to store the selected exhaust gas constituent(s) and the device""s instantaneous efficiency. When such device sulfation exceeds a critical level, the accumulated SOx must be xe2x80x9cburned offxe2x80x9d or released during a desulfation event, during which device temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the NOx device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of stored SOx.
Therefore, the inventors herein have recognized a need for a method and system for controlling the filling and purging of an emission control device with a selected exhaust gas constituent which can more accurately regulate overall tailpipe emissions of the exhaust gas constituent than prior art methods and systems.
In accordance with the invention, a method is provided for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from which is directed through an exhaust treatment system including an emission control device that stores an exhaust gas constituent during lean engine operation and releases previously-stored exhaust gas constituent during engine operation at or rich of stoichiometry. Under the invention, during lean engine operation, the method includes determining a value representing an incremental amount, in grams per second, of a selected exhaust gas constituent, such as NOx, present in the engine feedgas as a function of current values for engine speed, engine load or torque, and the lean operating condition""s air-fuel ratio. The method also includes determining a value representing the incremental amount of the exhaust gas constituent (e.g, NOx) being instantaneously stored in the device, preferably, as a function of device temperature, the amount of the constituent that is already stored in the device, an amount of sulfur which has accumulated within the device, and a value representing device aging (the latter being caused by a permanent thermal aging of the device or the diffusion of sulfur into the core of the device material which cannot be purged).
The method further includes calculating a value representing instantaneous tailpipe emissions of the exhaust gas constituent (e.g., NOx) based on the difference between the feedgas value and the incremental constituent-storage value; comparing the instantaneous tailpipe constituent emissions value to a predetermined threshold value; and discontinuing the lean engine operating condition when the instantaneous tailpipe constituent emissions value exceeds the predetermined threshold level, either instantaneously or as averaged over the course of a device purge-fill cycle, whose duration is determined by a timer which is nominally reset to zero upon commencement of an immediately prior rich engine operating condition.
In accordance with another feature of the invention, in a preferred embodiment, the method further includes generating a value representative of the cumulative number of miles that the vehicle has traveled during a given device purge-fill cycle; and determining a value representing average tailpipe constituent emissions in grams per mile using the instantaneous tailpipe constituent emissions value and the accumulated mileage value.
In accordance with another feature of the invention, an exemplary method further includes determining a need for releasing previously-stored exhaust gas constituent from the device; and deselecting the device-filling lean engine operation in response to the determined need. More specifically, under the invention, determining the need for releasing previously-stored exhaust gas constituent includes calculating a value representing the cumulative amount of the constituent that has been stored in the device during a given lean operation condition, based on the incremental constituent-storage value; determining a value representing an instantaneous constituent-storage capacity for the device; and comparing the cumulative constituent-storage value to the instantaneous constituent-storage capacity value. In a preferred embodiment, the step of determining the instantaneous constituent-storage capacity value includes estimating an amount of sulfur which has accumulated within the device.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.